[wp-trac] [WordPress Trac] #18836: ORDER BY RAND() is slow

[WordPress Trac]

#18836: ORDER BY RAND() is slow
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 Reporter:  scribu       |       Owner:
     Type:  enhancement  |      Status:  new
 Priority:  normal       |   Milestone:  Awaiting Review
Component:  Performance  |     Version:
 Severity:  minor        |  Resolution:
 Keywords:               |
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Description changed by scribu:

Old description:

> WP_Query currently accepts 'orderby' => 'rand' which translates to ORDER
> BY RAND().
>
> This is very slow when you have many posts, since it effectively calls
> RAND() for each row.
>
> A faster way would be to call RAND() only once and put it in the LIMIT
> clause.
>
> The only thing is that we have to make sure that the generated number is
> smaller than (total number of posts - number of posts to fetch).
>
> So, this would require to do an extra query to calculate the total. It
> should still be faster than the current method.
>
> If we want to get all the posts, we can get them in any order and then
> randomise their order using shuffle() in PHP.

New description:

 WP_Query currently accepts 'orderby' => 'rand' which translates to ORDER
 BY RAND().

 This is very slow when you have many posts, since it effectively calls
 RAND() for each row.

 A faster way would be to call RAND() only once and put it in the LIMIT
 clause.

 The only thing is that we have to make sure that the generated number is
 smaller than (total number of posts - number of posts to fetch).

 So, this would require to do an extra query to calculate the total. It
 should still be faster than the current method.

 If we want to get more than one post, we can get them in any order and
 then call shuffle() on the resulting array.

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Ticket URL: <http://core.trac.wordpress.org/ticket/18836#comment:3>
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